[commands] Extract common trigger binding logic (#7550)

This makes the logic clearer in the actual binding methods and will hopefully make it less annoying to make changes such as allowing control over initial edges.

Also changes Java to use previous and current to match C++.

wpilibNewCommands/src/main/native/include/frc2/command/button/Trigger.h

@@ -11,6 +11,7 @@
 #include <frc/event/EventLoop.h>
 #include <frc/filter/Debouncer.h>
 #include <units/time.h>
+#include <wpi/FunctionExtras.h>
 
 #include "frc2/command/Command.h"
 #include "frc2/command/CommandScheduler.h"
@@ -291,6 +292,13 @@ class Trigger {
   bool Get() const;
 
  private:
+  /**
+   * Adds a binding to the EventLoop.
+   *
+   * @param body The body of the binding to add.
+   */
+  void AddBinding(wpi::unique_function<void(bool, bool)>&& body);
+
   frc::EventLoop* m_loop;
   std::function<bool()> m_condition;
 };